The unnamed official said a 19-year-old man shot himself yesterday after police surrounded his home in the Timberlea suburb of Halifax, Nova Scotia.
A 23-year-old American woman from Geneva, Illinois, was arrested at Halifax Stanfield International Airport and confessed to the plot, the official said, adding that she had prepared a number of pronouncements to be tweeted after her death.
Two other Nova Scotian men have been arrested in connection with the plot, one aged 20 and the other 17.
The official said the suspects used a chat stream and were apparently obsessed with death and had lots of photographs of mass killings. Police and other officials said the plot was not related to Islamist terrorism.
Police said the suspects had access to firearms, but did not elaborate.
At the home of the male suspect, officers saw two people leave the house who they determined were his parents and pulled them over on a traffic check. They then called the suspect.
The man told police that he did not have any guns, but shot himself as he was on his way out of the house, the unnamed official said. Police worked with Canadian border officials to find the female suspect on her flight as she was making her way from Chicago.
Detectives said on Friday they first received information a day earlier about a potentially significant weapons-related threat. They confirmed that the two other Nova Scotia men were also involved, although their exact part in the plot is yet to be determined.
The police official said the 17-year-old was wanted for threatening to mount a gun attack on a high school and had an outstanding warrant.
Brian Brennan, commanding officer of Nova Scotia Royal Canadian Mounted Police, said: “I am extremely pleased to report that this threat was averted.
“Had they been able to carry out their intentions the possibility for a large loss of life was definitely there.”
He added: “We believe we have apprehended all known individuals in this matter and have eliminated the threat. We are not seeking any further suspects at this time in relation to this investigation.”
Brennan said he would describe the alleged plotters as a “group of individuals that had some beliefs and were willing to carry out violent acts against citizens but there’s nothing in the investigation to classify it as a terrorist attack”.
“I can tell you that it’s not culturally based,” he added.
“It was their intention to go to a public venue in the Halifax region on 14 February with a goal of opening fire to kill citizens, and then themselves,” the Royal Canadian Mounted Police said.
Canadian public safety minister Steven Blaney applauded the work of police in Canada and Geneva, Illinois as well as border officials in a statement.
He said the arrests were a “great example of the fine work they do on a daily basis to help keep Canadians safe”, Canada’s CBC News reports.
The Geneva police department said in a statement they were contacted by Canadian police and had no contact with the American woman before her arrest.
Nova Scotia premier Stephen McNeil said: “On behalf of all Nova Scotians, I thank our police officials, here and in the United States, for their quick and professional work.
“We are grateful for their dedication and vigilance.”
The names of the suspects have not been released.